The present invention relates to the treatment of evaporator condensates containing compounds, which are more volatile than water. In the method, the condensate is put into contact with vapor, whereby impurities contained in the condensate are transferred to the vapor stream. The method is referred to as stripping. According to the present invention, vapor generated in the flashing of evaporator condensates is used as stripping steam. The invention relates to evaporation plants, wherein the liquid to be evaporated, such as black liquor and effluent water, contains compounds which are more volatile than water.
The inorganic compounds of black liquor generated in cooking are recovered from the black liquor and the organic part is used as fuel. For the recovery, the black liquor is evaporated to a high dry solids content (70-85%) in a multieffect evaporation plant. Most typically the evaporation plant is operated so that the vapor produced in an evaporation effect operating at the highest pressure is used to heat an evaporation effect having the next highest pressure, while the liquor flows countercurrently in respect to the heating vapor. Typically the evaporation plant has 5-7 effects. An evaporation effect comprises one or more evaporation units using one and the same heating vapor supply.
In sulfate cooking of cellulose pulp, malodorous sulfur compounds are generated, such as methylmercaptan, dimethylsulfide and dimethyldisulfide, as the sulfide and hydrogen sulfide ions react with lignin or the methoxyl groups of lignin fragments. The cooking also releases abundantly of methanol. Vapors containing sulfur compounds and methanol are abundantly released e.g. in the evaporation of black liquor, in which said compounds are distilled and condensed into condensates of a multieffect evaporation plant. Methanol is the most significant of the volatile components of black liquor condensates, as it is the major source of the COD-value (CODxe2x80x94chemical oxygen demand) of the condensates due to its high concentration. Due to environmental requirements and in order to increase the reuse value of the condensates, methanol is removed from the condensates.
The aim is to treat condensates in such a way that said volatile components can be made to be concentrated into a small amount of foul condensate, which is usually purified by steam stripping. The condensates may be divided into a pure condensate and a condensate to be purified, e.g. by means of a certain construction of the heat transfer elements of the evaporation plant. U.S. Pat. No. 4,878,535 discloses one construction of this kind. The volume of foul condensate may vary freely within certain limits set by the design of the element, depending on the operator""s requirements for the purity of the pure condensate. Normally the amount is 10-20% of the total condensate volume of the evaporation plant.
Reuse of the heat of the secondary condensate is important for the heat economy of the mill. For that purpose, condensates coming from each evaporation effect are usually flashed in the steam flow direction to a next effect, in which the heat of the flash vapor is utilized. The flashing also reduces the amount of volatile impurities in the condensate.
Foul condensate is most usually purified by steam stripping. Usually a stripping column for foul condensate is integrated to the evaporation plant for minimizing the heat consumption of the stripping. The steam used for the stripping is either vapor generated in evaporation or vapor produced of stripped condensate in a reboiler by means of fresh steam. When the stripper for foul condensate is located between the first and second evaporation effects, the impurities-containing vapor exiting the stripper is led as heating steam to the second effect, into the black liquor preheater or both. The remaining stripper gases are led via a trim-condenser to combustion or methanol liquefaction.
A common goal for mills, such as pulp and paper mills, is an increasingly higher degree of closure of processes in order to reduce the amount of effluents released to the environment. A closed water circulation results in concentration of impurities, which may cause disturbances in the processes in the mill. That is why e.g. the condensates must be purified as required prior to reusing them e.g. as process water. The increase in volume of condensates to be purified requires larger stripping columns and larger steam volumes, thus decreasing e.g. the energy economics of the mill. The object of the present invention is to minimize these problems and to provide for a method, which in a simple and economical way reduces the volume of foul condensate generated in an evaporation plant and thus reduces the need for condensate purification. The method according to the invention provides for an essentially odorless, pure condensate that may be used in a production plant without additional purification instead of raw water.
The present invention relates to a method of treating condensates in an evaporation plant, in which e.g. an effluent liquid of a pulp or paper mill is evaporated in multiple effects and condensates generated in the evaporation are purified. A characteristic feature of the method is that unflashed condensates from at least two effects are combined and flashed together and that vapor obtained from the flashing is made to contact with a condensate stream to be purified in a stripping column, wherefrom the impurities-containing vapor and the purified condensate stream are discharged. In the following, the stripping column according to the invention is referred to as a flash vapor stripper.
Evaporator condensates are usually flashed between the evaporation effects, and the vapor generated therein is used for evaporation in a following effect. In the coupling according to the invention, condensates are not flashed between each effect separately, but before flashing condensates are combined and the thus combined condensates are flashed at a desired location, and the flash vapor is led into a lower part of the flash vapor stripper. This provides for an adequate amount of vapor for the stripping. The condensates led to the flashing must be cleaner than the condensate to be purified, in order to obtain the desired purification efficiency in the flash vapor stripper. For example, in a six-effect evaporation plant, where the stripper according to the invention is located between the fourth and fifth evaporation effects, the volume of vapor generated in the flashing of pure condensates is about 15% of the volume of condensate to be purified and led into the flash vapor stripper.
The flashing releases volatile impurities from the condensate into the flash vapor. Thus, the flashed condensate is in most cases without further treatment adequately pure to be used as process water. Subsequently, the flashed condensate and the condensate stripped in the flash vapor stripper may be combined for further use. If it is desired to combine the flashed condensate and the stripped condensate, the flashing may be performed at the bottom part of the flash vapor stripper, whereby a separate flashing vessel is not needed. In such a case, the evaporator condensate obtained from previous evaporation effects is led into a lower part of the flash vapor stripper, where a pressure decrease generates vapor to be used as stripping vapor.
From the bottom of the stripping column, the flash vapor travels through mass transfer trays or packings to the upper part of the column, wherefrom the vapor is led into a following evaporation effect. The most preferred location for the flashing and the flash vapor stripper is prior to the evaporation unit wherefrom the condensate to be stripped is fed into the stripper column. Thus, the condensate to be stripped is at a saturated temperature corresponding to the pressure in the stripper, and no stripping vapor is consumed for heating the condensate. If the amount of stripping vapor is 15% of the amount of the condensate to be purified, about 75% of the methanol contained in the condensate to be purified can be separated at a pressure of 40 kPa. In view of heat economy, it is preferred to lead the foul vapor obtained from the top of the flash vapor stripper to be used as heating vapor in an evaporation effect subsequent to the stripper. The evaporation unit, wherein the stripper vapor is led, must be provided with either a segregation of condensates into a pure and a foul condensate or a significant venting in order to prevent the condensate obtained from said unit from fouling caused by the stripper vapor. The stripper vapor may also be led into a separate vapor part in the evaporation unit or into a separate condenser, whereby the foul vapor obtained from the top of the stripper may be condensed into a fraction of its own and purified e.g. in a stripper for foul condensate.